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A review of the classic DC-3 and the Handley Page HP.42 yielded a surprisingly close match in empty and max gross take off weight at around 18,000 and 28,000 lbs for the HP.42 and 16,000 lbs and 25,000 for the DC-3. The HP.42 cruised at 87 knots on around 4x500 hp, and the DC-3 at 180 knots on 2x1200 hp. The HP.42 was a lower aspect biplane, the DC-3 a graceful mono plane, the epitome of aviation advances for that era but...

Questions: How much of the DC-3's speed increase was the result of improved engine/prop efficiency? How much from the DC-3's speed increase came from its beautiful Prandtl inspired wing? How much from retracting its gear? How much closer would the HP be with those improvements, while remaining a biplane (perhaps with improved struts)?

I do not expect an exact answer, I am more interested to see how the numbers were crunched and a short explanation why. Thanks.

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Struts every which way, uncowled engines, wheels with fenders hanging out, wires, bits sticking out here, bits sticking out there, corrugated skins, fabric wings and tail... it's a mess; a flying Truss Bridge. There's just too much to try to quantify it like that.

You could probably take a DC-3 and start adding all those things one at a time, and each item would knock off 10-20 kt, and eventually you'd end up down at 87 kt, but the slower you get the smaller each accumulative penalty would be.

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    $\begingroup$ True, oh John. I am going for rough percentages, particularly the engines. I am not sure if the HP.42 had variable pitch props. The transition from 4 to 2 engines echoes the trends today. Are we too infatuated with low drag wings? But the Prandtls on the HP or (please!) slats on the DC would be major improvements. Stall on the DC was controlled by aileron into the stall (hoping to stall the other wing) . Slats seem so much better. $\endgroup$ – Robert DiGiovanni Oct 11 '19 at 7:19
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    $\begingroup$ I think that implies that the performance difference was nearly all down to reduced form drag, and not improved prop efficiency. $\endgroup$ – Robin Bennett Oct 11 '19 at 11:08
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    $\begingroup$ The props are fixed wood ones, but if they are optimized for cruising, the difference between constant speed and fixed is not that great all else being the same. If optimized for climb then they are underpitched for cruise, and there is penalty at cruise of maybe 10-20 kt. If it had "climb" propellers and did 87kt, changing the props to ones with more pitch, "cruise" propellers, might add 10-20 kt, maybe. $\endgroup$ – John K Oct 11 '19 at 13:21
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In the case of biplane and a monoplane where all the wings involved have the same aspect ratio, the extra wingtips of the biplane should cause no loss of efficiency. On the other hand, interference between the top and bottom wings of the biplane will cause some loss of efficiency.

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